Plaster of paris casts to immobilize body members or limbs after fractures or breaks have been largely supplemented by synthetic casting tapes and bandages which employ polymeric materials in place of plaster of Paris. Polyurethane prepolymers, capable of being cured when reacted with water, have found wide use as replacement for plaster of Paris. The polyurethane prepolymers are coated on a fabric substrate to make the casting tape. The fabric substrate used in the synthetic casting materials has become to a large extent a fiberglass material. The fiberglass materials offer advantages in terms of strength of the finished cast and various constructions of fiberglass fabrics have been used as substrates for synthetic casting tapes. For the most part these fiberglass substrates have been Raschel knit fabrics. U.S. Pat. Nos. 3,686,725; 3,787,272 and 3,882,857 disclose specific fiberglass materials or the treatment of fiberglass materials to produce fiberglass substrates particularly suitable for use in orthopaedic casting tapes and bandages. U.S. Pat. No. 4,323,062 also disclose the cast substrate made from a combination of glass fibers and a second fiber such as cotton, flax, rayon, wool, acrylic, resin, nylon, or polyester.
Although fiberglass casting tapes have found extensive use in orthopaedic casting, one of the problems with fiberglass casting tapes is the tendency of the knitted fiberglass cast substrates to unravel on at least one end of the casting tape. As the tape is applied to the patient, there is one end of the tape which has a tendency to unravel and the unraveling causes the ends of the fiberglass substrate to become free and extend away from fabric substrate. As these free ends are coated with a prepolymer composition, they become stiff as the prepolymer cures and are like wire in their consistency. These free ends may be very uncomfortable for a patient if the wire-like end protrudes into the skin of the patient. The excessive raveling may also generate a considerable amount of loose strands in the casting tape which become unsightly and makes the casting tape difficult to apply. The unraveled end is not readily cut off from the bandage because of the nature of the prepolymer. The prepolymer is very sticky and attempting to cut the prepolymer coated substrate before curing is difficult. If the substrate contains only fiberglass it is possible to control the raveling problem to some extent by the use of high temperature heat treatment steps after the substrate is manufactured. However, the heat treatment does reduce the softness of the fabric and adversely alters the conformability of the casting tape. The raveling problem is particularly significant with highly conformable fiberglass substrates which contain elastic yarns. Fiberglass substrates containing elastic yarns, such as the substrate disclosed in co-pending application Ser. No. 873,648, filed June 12, 1986 now U.S. Pat. No. 4,668,583 cannot be heat treated because of the presence of the rubber or extensible filament in the substrate. The heat treatment would degrade the elastic yarn and the fabric would lose much of its elasticity.